How 4X4s Got So Good

It's the summer of 1974, and you're ready to kick up some dust on California's Rubicon Trail in your Jeep CJ-5. Hope you're ready for a workout. To engage the old Jeep's four-wheel drive, first you have to climb outside and lock the hubs on the front axle. Once that's finally done, hop back inside and pull the lever by your feet all the way back into 4WD Low (it'll take some arm strength). Now that the truck's ready, you'd better make sure you are too. If those tires began to slip on some wet rocks, you'll need to modulate the throttle and brakes while simultaneously sawing the steering wheel to hunt for spots where the tires can get a grip. It takes a lot of work to maintain forward progress.

Now fast-forward 40 years. This time you're four-wheeling the same legendary trail in a new Jeep Wrangler Rubicon. The driver can shift into 4WD without ever leaving the cab. When the tires lose their footing, electronic traction control will slow the spinning wheel and distribute torque to the wheels with the best grip. Need even more traction? Just push the buttons to engage the electronic locking differentials and disconnect that front swaybar—all from the comfort of the Jeep's climate-controlled cab.

Four decades of automotive advancements have brought huge leaps in four-wheel-drive capability. What was once a slow, analog process that required heaps of grit and know-how has been automated and made available at the push of a button. Still, that doesn't mean the core off-road driving skills are no longer important—or that tackling the trail is no longer challenging, or fun. In fact, because there are so many more tools at the driver's disposal, the person behind the wheel needs to learn not only those classic 4WD techniques, but get a crash course in current technology and when to use it.

It's a brave new world. Here's the trail that took us here.

Push-Button 4WD

Ford F-150 4WD selector.

Ford

Back in the 1970s, just shifting into 4WD was a multi-step process of twisting those hard-to-turn locking hubs on the front axle, then pulling the stiff and notchy lever mounted on the floor of the cab to engage four-wheel drive. All of this had to be done long before a trail ever becomes challenging. Wait too long and you might find yourself trying to lock those hubs with your boots in six inches of mud. Worse, the truck itself could be buried to the axles in muck. And engaging 4WD after you're stuck is no guarantee 4WD will get you unstuck.

By the 1980s, automatic locking hubs that could engage 4WD from the cabin became common, as did more complex central axle disconnect assemblies. Some of these early systems were unreliable. But by the early 2000s these systems had improved and allowed the transition from 2WD to 4WD to happen quickly, smoothly, and reliably at near-highway speeds. These days the ballooning market for crossovers has made all-wheel-drive very popular, so front axles that are designed to spin all the time, are often used on 4WD vehicles too with no vibration or effect on fuel economy.

Thanks to electronics, those bulky 4WD shifter levers have disappeared. They've been replaced by simple knobs or buttons that electronically shift the transfer case. So moving from high- to low-range 4WD requires no more muscle to operate, and takes up no more cabin real estate, than a power window switch. This drivetrain mode-switching can happen seamlessly without you ever leaving the seat. That's a clear win for technology. But the real leap came when modern electronics began managing traction.

Ram Power Wagon

Smart Systems

Traction control, like the system that's probably on the car in your garage, works by tapping into the anti-lock brake system (ABS). A typical four-channel ABS system uses a wheel speed sensor for each wheel, an ABS electronic control unit, and a hydraulic control unit to sense an impending brake lockup. If one or more wheel begins to lock, the system will modulate the brakes, preventing the lockup—and avoiding a skid. Traction control, which began to show up on passenger cars in the late 1980s, uses these ABS component to apply the brakes to one or more wheels if they lose traction and begin to spin. After braking a wheel or wheels, the system transfers torque across the axle to the wheel that does have grip.

Mainstream luxury 4WD vehicles began to employ four-wheel traction control systems in the late 1990s. Today, brake-based traction control is used on virtually every 4WD vehicle. It intervenes quickly to add a level of capability many older trucks never had.

On a steeply rutted climb, for instance, an older truck with open differentials would be stuck once the wheels lost traction. But traction control can halt a spinning wheel even if it's lifted high into the air and get things back on the right track. As long as the driver feeds in the right amount of throttle, these systems are powerful enough to pull a 4X4 up some nasty slopes.

Jeep Cherokee Trailhawk mode selector

Ben Stewart

The most sophisticated systems, like those in some Land Rovers, Jeeps, and Toyotas, tie the traction control and ABS brakes into other vehicle controls. These advanced systems can alter the throttle responsiveness, transmission shift points, stability control, and even the electronic differential locking operation, tailoring their responses to specific off-road conditions. In some cases, it's as easy as matching the pictogram below the 4WD knob to the trail ahead. For instance, the traction control becomes less restrictive in "sand" and "mud" modes, when more wheelspin is necessary to keep you moving forward. But in a "rock" mode, the truck might relax throttle responsiveness for easier crawling. Toyota even has a feature called "Crawl Control" that bundles these systems together with cruise control. Just select the appropriate speed (from five different settings) and Crawl Control will modulate the throttle and traction for you. All the driver has to do is steer.

Similarly, hill descent programs smoothly operate the brakes for you on steep declines. The best ones are adjustable and allow the driver to select the speed of vehicle and match that to the steepness of the hill. These systems can quickly brake and release each wheel individually—a level of downhill speed control that would have been nearly impossible just a few years ago.

And when the terrain is so severe even traction control won't help, electronic locking differentials can exponentially expand a vehicle's capability.

Diff Locking

Open differentials are designed to allow for the left and right wheels on an axle to turn at different speeds during cornering. On a right hand bend, for instance, the outside wheel must travel a longer distance than the inside wheel. So that outside wheel must turn at a greater speed. (Automakers made some dazzling movies in the 1930s to explain this technology, like so:)

An open differential allows this to happen smoothly. The problem is that on a slippery surface, an open differential sends the majority of its power to the wheel that turns with the least resistance. On an icy road or a muddy trail, once one wheel spins, the vehicle doesn't move.

Here's where it gets a little tricky. Limited-slip differentials are great devices to boost traction in the dirt because they can bias torque between the left and right wheels. But they never fully lock up enough to provide equal power to both wheels. The only device that can do that is a locking differential. As long as the wheels are traveling straight, a locker will split power equally between left and right. Put a locking differential on the front and rear axles of a 4WD vehicle and power is split equally to all four wheels—and the traction is incredible.

The problem is that lockers aren't fun on the street. When traveling around corners, that differential must unlock to allow for the difference in left and right wheel speeds. An old-style automatic locking differential typically unlocks with a bang and an unsettling wiggle in the chassis. That's why virtually no automaker offered these from the factory on production 4WD vehicles. But in the late 1990s some automakers began offering electronic locking differentials. When locked, both wheels on an axle provide equal and constant traction. At the push of a button, the diff unlocks and works like a conventional open differential for pavement use.

Toyota Tacoma TRD diff lock and crawl control.

Toyota

Today, these selectable lockers are optional on a wide variety of 4WD vehicles. The most sophisticated off-roaders, like some Land Rovers, offer an active locking differential that can lock and unlock smoothly and transparently when needed without any intervention from the driver. These sophisticated smart lockers help ease the load on the traction control system. Together, create an incredible level of capability.

Suspension Adjustability

Like differentials, suspension systems for 4WD vehicles used to be a compromise: Those that perform well on the street typically don't do well off-road. In the old days, the long-travel springs, shocks, and swaybars designed for a 4WD vehicle would end up being sloppy on the street. Thanks to new technology, however many of today's best 4WD vehicles can have excellent performance on dirt and pavement.

Jeep Grand Cherokee air suspension

Since 2003, the Jeep Wrangler Rubicon, the icon of off-roading, has not only used selectable locking differentials but also an electronically disconnecting front swaybar. A swaybar, or stabilizer bar, is something that attaches to the vehicle's frame as well as each side of the suspension. As a vehicle drives through a corner, the bar twists to limit the body roll. That level of suspension control is great for the street, but a swaybar designed for street performance can limit how freely the suspension moves off-road.

With an electronically disconnecting swaybar, one press of the dash-mounted button will uncork an incredible amount of front wheel travel and axle articulation. More wheel travel means the tires can maintain contact with the trail over the roughest spots. Press that button again and the swaybar would re-attach, providing the roll control needed for street use. Toyota has an even more advanced system on SUVs like the 4Runner and Land Cruiser called Kinetic Dynamic Suspension System (KDDS). This system automatically disconnects the swaybars when more axle articulation is needed. There's no need even to press a button.

Similarly, the tech behind adjustable air suspension allows for better performance on both street and trail performance. On many Land Rover products, for instance, the long-travel independent suspension uses adjustable air springs that can vary height and ground clearance to suit the conditions. That allows the vehicle to ride at a more stable, even lowered height on the road and then, at the push of a button, raise up to provide trail clearance that in some cases rivals that of a modified vehicle. That level of reliable on-demand adjustability simply wasn't possible years ago.

Opening the Trail

Land Rover

It's not just that newer electronic 4WD systems are smarter and more capable than the simpler ones that came before them. This technology has been democratized, too—it's available on relatively inexpensive vehicles. A 2016 F-150 4X4 comes standard with four-wheel traction control. An electronic locking rear differential only cost $325. That's a steal. These systems can take showroom 4X4s places that would have left their predecessors of decades past stuck in the mud.

Perhaps the greatest benefit is the end to trade-off. Since all this electronic 4WD hardware doesn't affect street drivability, these off-roaders are just as happy on road as they are off it.

A Part of Hearst Digital Media
Popular Mechanics participates in various affiliate marketing programs, which means we may get paid commissions on editorially chosen products purchased through our links to retailer sites.